Hack an old Boombox to give line in
The execution’s a bit sloppy and as some commenters have pointed out, he could probably have just used the DIN socket, but the concept’s good:
Hacking a line-in socket into an 80s radio and cassette player
As well as allowing connection of phones and other media players, it will allow playback of the PS-F5/9 through the boombox speakers. A relatively inexpensive way of getting good quality sound (good quality boomboxes can be had for some very reasonable prices on eBay) with the bonus of keeping your kit in the right decade 😉